An image sensor is a semiconductor device for converting an optical image into an electrical signal. The image sensor is roughly classified as a charge coupled device (CCD) image sensor or a complementary metal oxide semiconductor (CMOS) image sensor (CIS).
In an image sensor, a photodiode is formed in a substrate having readout circuitry using ion implantation. As the size of a photodiode reduces more and more for the purpose of increasing the number of pixels without an increase in a chip size, the area of a light receiving portion reduces, so that an image quality reduces.
Also, since a stack height does not reduce as much as the reduction in the area of the light receiving portion, the number of photons incident to the light receiving portion also reduces due to diffraction of light, called airy disk.
As an alternative to overcome this limitation, an attempt of forming a photodiode using amorphous silicon (Si), or forming a readout circuitry in a Si substrate and forming a photodiode on the readout circuitry using a method such as wafer-to-wafer bonding has been made (referred to as a “three-dimensional (3D) image sensor). The photodiode is connected with the readout circuitry through a metal line.
Meanwhile, a bonding surface of a wafer including a circuitry has two kinds of layers including material for a line and an interlayer dielectric at a wafer-to-wafer bonding interface. Because the materials of the interface are not uniform (same material at surface) according to a related art, the bonding force of the bonding surface may reduce, and accordingly, a dark current may be generated.
Also, according to a related art, a dark defect may be generated in a photodiode by the etching of a trench in a shallow trench isolation (STI) process for device isolation between pixels, resulting in incomplete device isolation between pixels.
According to a related art, since both the source and the drain at the sides of the transfer transistor are heavily doped with N-type impurities, a charge sharing phenomenon occurs. When the charge sharing phenomenon occurs, the sensitivity of an output image is reduced and an image error may be generated. Also, according to a related art, because a photo charge does not swiftly move between the photodiode and the readout circuitry, a dark current may be generated or saturation and sensitivity reduce.